Specificity of the activation of [³H]hemicholinium-3 binding by phospholipase A₂

Phospholipase A₂ (PLA₂) treatment has been shown previously to stimulate the sodium dependent high-affinity choline uptake system as assessed by both the specific binding of [³H]hemicholinium-3 ([³H]HCh-3) and the uptake of [³H]choline. In the present study, the specificity of PLA₂-induced stimulation upon [³H]HCh-3 binding has been examined. PLA₂, as well as phospholipase C (PLC), treatment of synaptic membranes produced a dose-dependent increase in the specific binding of [³H]HCh-3 whereas neither phospholipase B nor phospholipase D had any effect. PLC-induced stimulation of [³H]HCh-3 binding resulted from a significant decrease in the K(d) without a change in the maximum binding of [³H]HCh-3 binding. PLC treatment of synaptosomes resulted in an inhibition of [³H]choline uptake accompanied by an inhibition of Na⁺, K⁺-adenosine triphosphatase activity. In contrast to the increase of [³H]HCh-3 binding, the specific binding of both [³H]desipramine and [³H]mazindol was decreased by PLA₂ treatment. After PLA₂ treatment, [³H]HCh-3 binding was increased about 2.5-fold over basal levels in different regions of the brain. Electrolytic lesions of the medial septal nucleus and kainic acid-induced lesions of the striatum resulted in a marked reduction of [³H]HCh-3 binding in the hippocampus and the striatum, respectively. Residual [³H]HCh-3 binding in the denervated hippocampus and lesioned striatum was increased by PLA₂ treatment but remained lower than that in PLA₂-treated controls. Finally, atropine-induced up-regulation of [³H]HCh-3 binding in vivo was not additive with PLA₂-induced stimulation. These results support the hypothesis that PLA₂ might be involved in the regulation of the sodium-dependent high-affinity choline uptake.